Absorption refrigerator and production method thereof

ABSTRACT

An absorption refrigerator includes a high temperature regenerator for heating a water solution including a halogen compound to generate steam. An oxide film having thickness of 0.02–5.0 μm, one of the colors blue, purple and gray, or a hydroxyl group is formed on a surface of at least one of the high temperature regenerators and the heat exchanger.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of Ser. No. 09/838,139, filed Apr.20, 2001 now U.S. Pat. No. 6,813,901, which is a continuation of Ser.No. 09/362,559, filed Jul. 28, 1999, now U.S. Pat. No. 6,279,342, whichis a continuation of Ser. No. 09/100,254, filed Jun. 19, 1998, now U.S.Pat. No. 5,964,103, which is a continuation-in-part of Ser. No.08/721,720, filed Sep. 27, 1996, now U.S. Pat. No. 5,806,337 thecontents of each of which are incorporated herein in their entireties.

BACKGROUND OF THE INVENTION

The present invention relates to a novel absorption refrigerator and,more particularly, to an absorption refrigerator which has excellentcorrosion resistance properties, with main structural components of theabsorption refrigerator being highly protected from corrosion by formingin advance corrosion protective films on surfaces of the main structuralcomponents, and the invention relates to a method of manufacture of theabsorption refrigerator.

Absorption refrigerators each use a rich LiBr solution as an absorptionsolution and water as a refrigerant. In the absorption refrigerator, ingeneral, the higher the concentration of the LiBr solution is, thehigher the efficiency of refrigeration becomes, so that theconcentration and temperature of the LiBr reach to 65% and 160° C.,respectively, at the highest temperature portion of a double effectabsorption refrigerator, for example. Under such circumstances, thestructural members of the refrigerator tend to become seriouslycorroded. Therefore, a suitable inhibitor, such as tungstate, molybdatehas been added to the solution, as disclosed in JP A 58-224186 and JP A58-224187, whereby corrosion has been reduced. The inhibitor is usedtogether with the hydroxide of an alkaline metal, which is a pH adjusterand forms corrosion protective films on the members due to the oxidizingforce thereof, whereby corrosion is suppressed.

Apart from a method of forming a corrosion protective film duringoperation of a refrigerator, there is a method in which, as disclosed inJP A 1-121663, JP A 2-183778, in order to form a corrosion protectivefilm on an inner wall of a high temperature regenerator, in contact witha most highly corrosive absorption solution therein before operation ofthe refrigerator, a film forming liquid recirculation line and arefrigerant supply line are provided in the high temperatureregenerator, and a film coating operation is carried out byrecirculating a film forming liquid, heated and concentrated in the hightemperature regenerator, through the film forming liquid recirculationline, whereby corrosion protective films are coated on the inner wall ofthe high temperature regenerator and the surfaces of the piping of therecirculation lines which are in contact with the absorption solution.

Further, as a corrosion protective film coating method which does notuse an absorption solution, there is a method in which a corrosionprotective film is formed in the interior of a refrigerator by heatingit to 400° C. or more under a gas atmosphere in which the dew pointthereof is controlled so that the partial pressure of steam becomes 10ppm or less and the partial pressure of oxygen is adjusted to about 10Pa–10 kPa, as disclosed in JP A 6-249535.

As for a method of forming a corrosion protective film using aninhibitor during operation of a refrigerator, in a case wherein chromateand nitrate are used as the inhibitor, it is feared that pitting mayoccur in the structural material when the concentration of the inhibitorreaches a certain level or more, and so there remains a problem ofmanagement of the inhibitor concentration when an inhibitor is used. Onthe other hand, since molybdate has a low solubility to LiBr and theoxidation is weak, there remains a problem in that much time is requiredto form a stable corrosion protective film, and the refrigerationefficiency decreases due to generation of hydrogen gas during formationof a stable corrosion protective film, whereby it is difficult to attaina sufficient corrosion protective effect. Further, a relatively largeamount of inhibitor is consumed by the time a stable corrosionprotective film is completed, so that it is necessary to add inhibitor.

As disclosed in the above-mentioned prior methods, there is a method offorming a corrosion protective film by performing a film formingoperation using a film forming liquid recirculation line beforeoperation of the whole refrigerator, in order to solve theabove-mentioned problems. In this method, however, since the filmforming liquid used in the film forming operation is a LiBr solutionincluding molybdenum, as used in the method of forming a corrosionprotective film during operation of the refrigerator, the problem of adecrease in refrigeration efficiency due to generation of hydrogen gasduring the operation can be solved, however, other problems are stillleft unsolved in that much time is required for forming a corrosionprotective film due to the low solubility of molybdate, the fact thatthe oxidation also is weak, and in that much inhibitor is consumed.

The corrosion protective film formation method which does not use anabsorption solution and is disclosed in the above-mentioned prior artcan solve the problem that much time is required to form a corrosionprotective film because of the low solubility of molybdate and the weakoxidation, and the problem that a relatively large amount of inhibitoris consumed. However, the step of injecting an inert gas in order tocontrol the dew point, the step of condensing steam to lower thepressure to a prescribed level or lower, the step of injecting oxygengas to a prescribed pressure, etc. are needed, so that the corrosionprotective film forming method becomes not only complicated, but variousapparatuses are required, such as a vacuum pump, a pressure gauge, amass analyzer, a cold trap, etc., and so the refrigerator becomescomplicated in construction and high in cost. Further, in a method inwhich an interior of a refrigerator first is filled with an inert gas(since the inert gas is introduced after reduction of the pressure, theinert gas is replaced with gas in the entire interior of therefrigerator) and then oxygen gas is injected, even if oxygen is causedto flow therein, the oxygen gas does not enter all gaps and convectionportions in the interior of the refrigerator, so that it is difficult tomake the partial pressure of oxygen uniform in the refrigerator.Therefore, in some portions, a corrosion protective film is formed byexcessive oxygen, but some other portions lack oxygen, so that asufficient corrosion protective film is not formed or an incompletecorrosion protective film is formed, and so corrosion is not suppressed.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an absorptionrefrigerator in which reduction of the refrigeration efficiency due togeneration of hydrogen gas during the operation thereof is prevented,and in which a high corrosion resistance due to the provision ofcorrosion protective films is attained, such as by use of thin anduniform corrosion protective films of high corrosion resistance, formedon surfaces of the absorption refrigerator in an easy manner beforeoperation thereof, and it is an object to provide a method ofmanufacturing such an absorption refrigerator.

The above-mentioned object can be achieved by forming a corrosionprotective film on an absorption refrigerator by bringing hightemperature steam or air having an arbitrary dew point into contact withstructural material of the absorption refrigerator before operation ofthe refrigerator.

More specifically, the object is achieved by provision of an absorptionrefrigerator having a corrosion protective film formed by bringing steamof 200–800° C. (preferably 330–500° C., more preferably 350–450° C.) orair having an arbitrary dew point, into contact with structural materialof the absorption refrigerator before operation of the refrigerator, amethod of forming a corrosion protective film by causing theabove-mentioned steam or air having an arbitrary dew point to contactthe structural material of the absorption refrigerator, and an apparatusfor introducing the above-mentioned steam or air of an arbitrary dewpoint for practicing the method. The method of bringing theabove-mentioned steam or air having an arbitrary dew point into contactwith structural material of the absorption refrigerator can be achievedby a method of preventing corrosion by providing a gas introductioninlet in the absorption refrigerator and introducing the above-mentionedsteam or air of an arbitrary dew point through the gas introductioninlet, and by forming a corrosion protective film on a surface of eachpart of a high temperature regenerator, high temperature heat exchanger,low temperature heat exchanger, etc. by exposing the surface of eachpart to an atmosphere containing the above-mentioned steam or air of anarbitrary dew point, and then assembling the parts having the surfaceswhich have corrosion protective films formed thereon.

The present invention resides in an absorption refrigerator which useswater as a refrigerant and a halogen compound as an absorption solution,and which is characterized in that an oxide film having a thickness of0.02–5.0 μm, preferably 0.1–2.5 μm, more preferably 0.3–2.0 μm, and anycolor of blue, purple, black and gray, or a hydroxyl group, is formed ona surface of at least one of a heat exchanger and a high temperatureregenerator, or all the surfaces, contacting the above-mentionedabsorption solution, of the iron or iron containing structural materialconstituting the refrigerator.

Further, the present invention is characterized in that an oxide filmformed on a surface, in contact with an absorption solution, made ofiron or iron-containing components constituting the absorptionrefrigerator, is thinner than an oxide film formed on a surface of aniron or iron-containing component constituting piping forming a coolingwater line with a cooling tower, a cold water line and a steam line.

Further, the present invention resides in an absorption refrigeratorcomprising a high temperature regenerator for heating a water solutionhaving a halogen compound to generate steam, a condenser for condensingthe steam, a low temperature regenerator for cooling the steam, anevaporator for evaporating the water from the condenser and producingcold water, an absorber for absorbing the water from the evaporator intoa water solution including a high concentration halogen compound, and aheat exchanger for returning the refrigerant issued from the absorber tothe high temperature regenerator and effecting heat exchange between thewater from the low temperature regenerator and the refrigerant from theabsorber, characterized in that an oxide film having thickness of0.02–5.0 μm, and any color of blue, purple, black and gray, or ahydroxyl group, is formed on a surface of at least one of the hightemperature regenerator and heat exchanger, or on at least a portion incontact with the water solution, steam or water and made of iron or aniron-containing material, of the high temperature regenerator,condenser, low temperature regenerator, evaporator, absorber and heatexchanger.

In accordance with the present invention, it is possible to form theabove-mentioned corrosion protective film, such as an oxide film on eachof the parts constituting each apparatus, other than the hightemperature regenerator and heat exchanger, and then to assemble theminto a whole apparatus.

The present invention resides in a method of production of an absorptionrefrigerator, which uses water as a refrigerant and a halogen compoundas an absorption solution, characterized by oxidizing a surface of atleast one of the heat exchanger and the high temperature regenerator ata temperature of 200–800° C., and adjusting the heating temperature andthe heating retaining time so that the value of a parameter (P),obtained according to P=T(5+log t), is 3.5–6.0×10³, preferably4.0–5.5×10³, and more preferably 4.4–5.0×10³, wherein T represents theheating temperature (°K), and t represents the heating retaining time(minute), or by heating the surface of at least one of the heatexchanger and the high temperature refrigerator in an oxidizingatmosphere in which the partial pressure of steam is 0.0001 or more andthe partial pressure of oxygen is 0.2 or more, thereby to form an oxidefilm thereon.

The present invention resides in formation of an oxide film on one ofthe heat exchanger and the high temperature regenerator, or on at leasta portion thereof in contact with the water solution, steam or water andwhich is made of iron or iron-containing material, of the hightemperature regenerator, condenser, low temperature regenerator,evaporator, absorber and heat exchanger, or in producing individuallyeach of the structural components.

In the absorption refrigerator according to the present invention,lithium bromide is used as the halogen compound for forming anabsorption solution of the absorption refrigerator, and so corrosion isproduced by the lithium bromide due to adhesion of bromine ions of highconcentration on the structural material. In general, the corrosion rateis faster at an initial stage and it decreases with passage of time.This is because an oxide film is formed on a surface of the materialwith the passage of time, and the oxide film works as a corrosionprotective film. That is, when a corrosion protective film is formed,the diffusion of water, oxygen, oxygen ions, iron ions, etc., whichinfluence corrosion, is suppressed by the corrosion protective film.Therefore, by forming in advance a corrosion protective film on asurface of the structural material by pre-oxidation, direct adsorptionand contact of bromine ions onto the surface of the structural materialcan be prevented, whereby corrosion can be prevented. Iron oriron-containing material is used for many of the structural materials.Iron oxide obtained by directly oxidizing the structural material can beused for a corrosion protective film. The corrosion resistance of thecorrosion protective film is influenced by not only the chemicalproperty of the film but by the physical property of the film as well.That is, as mentioned above, since the corrosion protective filmsuppresses diffusion of substances influencing corrosion, as mentionedabove, a corrosion protective film which is denser has a higherdiffusion suppressing ability. Even if the oxide film is made thicker,the corrosion resisting effect by the oxide film is small withoutsufficient denseness in the oxide film. Further, in the case where ahydroxyl group exists on the surface, a strong hydrogen binding occursin the film by protons of the hydroxyl group and a film of highcorrosion resistance is formed.

As methods of forming such a corrosion protective film, there are amethod of using high temperature water and a method of using hightemperature gas. In the case of high temperature water, a problem occursin that a high temperature and high pressure vessel is needed, thus alarge sized equipment is necessary. On the contrary, in the case whereair (partial pressure of steam is controlled, air of an arbitrary dewpoint) or steam is used as the high temperature gas, since the oxygenpotential and partial pressure of the steam can be controlledarbitrarily, the equipment becomes simple and the method can be easilyput into practice. Further, since a refrigerator or its components in anair atmosphere are oxidized as they are, there is an oxygen potential tothe partial pressure of oxygen in the air, at least, so that oxide filmformation does not become incomplete because of lack of oxygen. Further,it is important to make the oxide film which is formed in suchatmosphere very thin, for example, about several ten thousands angstrom,whereby an oxide film of high density can be obtained, allowing it towork as a corrosion protective film of high corrosion resistance.

In the case where stainless steel or a low alloy steel is used for thestructural material, also, the above-mentioned high temperature gas canbe used as the pre-oxidizing means in the same manner as describedabove.

Further, in case where the oxidation is performed before operation ofthe refrigerator, that is, in case of pre-oxidation treatment beingperformed, since a film is formed in the pre-oxidation treatment, theinhibitor is almost not consumed at all even in an initial stage, sothat it is unnecessary to supplement the inhibitor. By forming thecorrosion protective film before operation of the refrigerator, it isunnecessary to worry about corrosion of the structural material from aninitial stage of operation of the refrigerator, and it is possible toenhance the reliability of the product.

In the method of forming a relatively thick corrosion protective film,using gas as disclosed in the above-mentioned prior art, once an inertgas is filled, and then oxygen gas is filled, because separation occursin the corrosion protective film being formed on the surface to beprotected from corrosion, the film becomes nonuniform. On the otherhand, in accordance with the present invention, as mentioned above, arelatively thin film is formed. In this method, high temperature steam,air of an arbitrary dew point or oxygen gas is introduced into a portionin which air was previously filled, so that the oxygen partial pressurein the atmosphere exists to a greater extent than the oxygen partialpressure in the air, at least, and it does not occur that the corrosionprotective film is not formed due to lack of the oxygen partialpressure. Therefore, a uniform corrosion protective film can be formed.

As an absorption solution, a solution comprising, by weight, lithiumbromide 50–70%, alkaline metal hydroxide 0.05–1%, molybdate 10–150 ppmas Mo0₄ ²⁻, nitrate 5–350 ppm as NO₃ ⁻, higher alcohol 0.2–3%, andbalance water 30% or more is preferable.

An absorption refrigerator may be classified according to power source,use and construction, as expressed in TABLE 1. Except for a small amountof electric power for driving auxiliary apparatuses, it is almost notnecessary to use electric power. As a power source, many kinds ofsubstances, such as gas, oil or steam, can be used.

Basically, it is an object for the absorption refrigerator to producecold water for cooling, but the apparatus can be used for variousobjects, such as production of hot water for heating in winter, orsynchronized utility of cold water and hot water by one apparatus.

Since the absorption refrigerator uses water as a refrigerant and alithium bromide solution as an absorption solution, the pressure withinthe refrigerator during operation thereof is at atmospheric pressure orless, and so the refrigerator can not be a pressure vessel. Therefore,as for an operation qualification, there is no need for an operationqualification particular to the absorption refrigerator.

As for rotating parts, only small capacity pumps are needed forrecirculation of refrigerant and solutions, so that the refrigerator hasadvantages, such as less noise and less vibrations, as compared with anymechanical refrigerator.

A typical absorption refrigerator, apparatuses and devices of a directdouble effect absorption cold-hot water unit will be explainedhereunder.

The unit comprises a main body in which four heat exchangers of anevaporator, an absorber, a condenser and a low temperature regeneratorare incorporated into one shell, a high temperature regenerator, arefrigerant pump and a solution pump for recirculating a refrigerant andsolutions in the apparatus, respectively, and a solution heat exchangerfor effecting heat exchange between solutions.

TABLE 1 Article Name (AN) 1 Gas direct boiling cold hot water unit 2 Oildirect boiling cold hot water unit 3 Small sized cold hot water unit 4Steam double effect absorption refrigerator 5 Steam single effectabsorption refrigerator 6 High temp. water single effect absorptionrefrigerator 7 Low temp. water single effect absorption refrigerator 8Exhaust gas used absorption refrigerator cold hot water unit 9Absorption heat pump An Power source Use Construction Ref. capa. 1 Gas(city gas, natural Air-con. Dou. effect 80-1,650RT gas, etc.) 2Kerosene, special A oil Air-con. Dou. effect 80-1,650RT fuel, etc. 3 Gasor oil Air-con. Dou. effect 20-75RT 4 Steam of 5–8 kg/cm²G Cooler Dou.effect 30-1,650RT 5 Steam of 1 kg/cm²G Cooler Sin. effect 80-1,350RT 6100–150° C. hot water Cooler Sin. effect 80-1,350RT 7  75–99° C. hotwater Cooler Sin. effect 40-675RT 8 Various kinds of gases Air-con. Dou.effect 30-1,650RT (above 250° C.) 9 Gas, steam, etc. Process Sin. effectHeat amount Heater Dou. effect 3 × 10⁵–5 × 10⁷ kcal/h Note; AN-Articlename; Ref. capa.-Refrigeration capacity; Air-con. - Air conditioner;Dou. - Double; Sin.-Single.

In the evaporator, water as a refrigerant is sprayed over a bundle oftubes under a vacuum of about 1/100 atmospheric pressure, and, at thistime, evaporation heat is received from the cold water flowing in thebundle of tubes to evaporate the refrigerant, so that the cold water inthe bundle of tubes is cooled, whereby the apparatus is to be used foran object of cooling air.

In the absorber, since a lithium bromide solution cooled to a propertemperature by the cold water flowing in the tubes is at a saturatedpressure a little lower than in the evaporator, refrigerant vaporgenerated in the evaporator flows into the absorber to be absorbed inthe solution. The dilute solution which absorbs the refrigerant to bediluted is divided into two flows which are transferred into the hightemperature regenerator and the low temperature regenerator,respectively. The divided and diluted solutions are heated andconcentrated in the high and low temperature regenerators, respectively,to become concentrated solutions, and then are returned to the absorberagain.

In the high temperature regenerator, a heat source, such as gas or oil,is supplied from outside and burned in a furnace. The dilute solution isconcentrated by this heat, and vapor generated incidentally at this timeis passed through tubes of the low temperature regenerator and utilizedas a heating and concentrating source for the low temperature generator.

Vapor generated outside the tubes of the low temperature regenerator isliquified in the condenser and returned into the evaporator, whereby onecycle is completed.

Classifying roughly the method of transferring the diluted solution fromthe absorber into the two regenerators, there are two ways, one of whichis a parallel flow type in which the solution is divided into two andtransferred in parallel, and the other is a series flow type in whichthe solution is sent into the two regenerators in series. In the former,the operation pressure in the high temperature regenerator duringoperation is lower as compared with that in the latter and larger inmargin to the atmospheric pressure, so that it is easy to operate theapparatus, and the amount of the solution recirculating in the cycle issmall, so that it has an advantage that a heat exchange of small size issufficient for attaining the same efficiency.

Direct boiling double effect absorption type cold hot water unit:

The unit is a main heat source apparatus which is widely used forcooling and heating in general buildings. The unit is used for energyconversion, serving in place of an electric power source at the peak ofelectric power demand in summer. Since energy saving also is desired,there is a special energy saving type unit in which the efficiency israised by recovering energy of combustion exhaust gas in the hightemperature regenerator.

Further, the unit of this type has the widest application fields, and itcan be employed for a unit commonly used for both solar heat and directboiling type in which solar heat is applicable, a switching combustiontype in which fuel can be switched between gas and oil, an outdoor type,a cold hot water simultaneous supply type, etc.

Steam double effect, steam single effect absorption refrigerator:

In the double effect steam boiling apparatus, steam at a pressure of 8kg/cm²G is used. However, there is a low pressure steam double effectabsorption refrigerator which is operable at a pressure of 5 kg/cm²G,using the advantage of parallel flows, as mentioned above, or atpressure of 2 kg/cm²G, selecting a suitable temperature of cooling waterfor cold water.

Further, in a factory having an extra amount of steam of 1 kg/cm²G, asingle effect absorption refrigerator is effective.

Exhaust gas-used absorption refrigerator, cold hot water unit:

This is a refrigerator which uses as a heat source exhaust gas from adiesel engine or gas turbine, or exhaust gas from various kinds offactories. The exhaust gas also can be used for a double effectrefrigerator if the temperature is 250° C. or higher.

As for construction of the apparatus and devices thereof, a hightemperature regenerator portion of the double effect absorptionrefrigerator type hot water unit can be replaced by an exhaust gas heatrecovery apparatus. The exhaust gas heat utilizing portion can bereplaced by other heat source, such as steam, gas, oil, etc.

Solar absorption refrigerator:

Since the temperature of hot water heated by solar heat is about 85° C.,which is relatively low, it is used for a single effect typerefrigerator. However, since the heat source is unstable, it isnecessary to provide some type of back up system. A solar heat jointlyused direct boiling absorption refrigerator is most suitable therefor.When solar heat is insufficient, the direct boiling double effectrefrigerator can back up it, so that it is most advantage in equipmentcost and operation cost.

Absorption heat pump:

The absorption refrigerator is a machine for pumping heat from a lowtemperature heat source to a high temperature portion using some powersource, and the refrigerator uses heat absorption in the low temperatureportion, although, heat release in the high temperature portion can beused. As for heat balance in the refrigerator, since the sum of the heatamount of power required for pumping heat and the heat amount pumpedfrom the low temperature portion is equal to the amount of the heatrelease at the high temperature portion, it can be improved by 1.5–2times as compared with the case in which the power source is simply usedfor heating. This is an absorption type heat pump, which can be usedmainly for a factory process, a heating arrangement using pre-heatedboiler feed water.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an absorption refrigerator systemforming an embodiment of the present invention;

FIG. 2 is a graph showing the influence of the atmospheric oxidationtemperature on the corrosion amount of SS400 in a 65% LiBr solution withan inhibitor;

FIG. 3 is a graph showing the influence of the thickness of apreoxidation film on the corrosion amount in a 65% LiBr solution with aninhibitor;

FIG. 4 is a graph showing the corrosion behavior of preoxidized SS400 ina 65% LiBr solution with an inhibitor;

FIG. 5 is a graph showing the influence of the Li₂MoO₄ concentration onthe corrosion amount of SS400;

FIG. 6 is a graph showing the influence of the LiOH concentration on thecorrosion amount of SS400;

FIG. 7 is a diagram showing an X-ray diffraction pattern of SS400preoxidized in an atmosphere at 300° C. before and after a corrosiontest in a 65% LiBr solution with an inhibitor at 160° C.;

FIG. 8 is a diagram showing an X-ray diffraction pattern of SS400preoxidized in an atmosphere at 400° C. before and after a corrosiontest in a 65% LiBr solution with an inhibitor at 160° C.;

FIG. 9 is a diagram showing an X-ray diffraction pattern of SS400preoxidized in an atmosphere at 500° C. before and after a corrosiontest in a 65% LiBr solution with an inhibitor at 160° C.;

FIG. 10 is a diagram showing AES depth profiles for SS400 preoxidized inan atmosphere at 400° C. before and after a corrosion test in a 65% LiBrsolution with an inhibitor at 160° C.;

FIG. 11 is a graph showing hydrogen generation with the passage of timein an absorption refrigerator according to the present invention and ina conventional apparatus;

FIG. 12 is a graph showing the remaining amount of inhibitor with thepassage of time in an absorption refrigerator according to the presentinvention and in a conventional apparatus;

FIG. 13 is a schematic diagram of an apparatus for effecting oxidationby steam according to the present invention;

FIG. 14 is a perspective view of a plate heat exchanger;

FIG. 15 is a sectional view showing the interior of the heat exchangerin FIG. 14;

FIG. 16 is a flow chart of a process of production of a heat exchangerused in the absorption refrigerator shown in FIG. 1;

FIG. 17 is a graph showing characteristic curves of temperature-time inthe case of oxidation according to the present invention;

FIG. 18 is a graph showing a relation between the thickness of an oxidefilm and oxidation temperature;

FIG. 19 is a graph showing the suppressing effect of preoxidation on thecorrosion of SPCE plates in a 65% LiBr solution with an inhibitor;

FIG. 20 is a graph showing the suppressing effect of preoxidation on thecorrosion of SPCE plates in a 65% LiBr solution with an inhibitor;

FIG. 21 is a graph showing the suppressing effect of preoxidation on thecorrosion of SPCE plates in a 65% LiBr solution with an inhibitor;

FIG. 22 is a diagrammatic sectional view of a high temperatureregenerator;

FIG. 23 is a perspective view of a pipe in an electric furnace forexplaining production of piping according to the present invention usingthe electric furnace;

FIG. 24 is a diagrammatic view for explanation of the arrangement of aheater in piping, concerning the present invention;

FIG. 25 is a graph showing the relation between corrosion amount andtime in an apparatus according to the present invention and in aconventional apparatus;

FIG. 26 is a front view of an absorption refrigerator;

FIG. 27 is a left side view of the absorption refrigerator in FIG. 26;

FIG. 28 is a right side view of the absorption refrigerator in FIG. 26;

FIG. 29 is a perspective view of the whole absorption refrigerator;

FIG. 30 is a schematic view showing a system of a cooling water line anda cold water line; and

FIG. 31 is a schematic view showing a system of a steam line.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiment 1

Structural rolled steel plate (SS400) of 2 mm thickness was polishedwith emery paper up to #6/0, washed with ultrasonic vibration in acetoneand then subjected to a test. The test pieces had the following chemicalcomponents, by weight %:

C;0.03–0.13% (preferably 0.04–0.08%), Si;≦0.5%

(preferably≦0.05%), Mn;≦0.5% and Fe;bal. One practical example is asfollows:

C;0.05, Si;<0.01, Mn;0.24, P;0.016, S;0.010 and Fe;bal.

This material has a yield strength at room temperature of 25 kg/mm² ormore, a tensile strength of 41–52 kg/mm² and an elongation of 21% ormore.

Further, for corrosion estimation of a welded portion, a laminate of twosheets of JIS SPCE cold rolled steel plate, joined by plasma welding attheir peripheral portion, was used as a test piece. Atmosphericoxidation conditions were set to 300° C.(1 h), 400° C.(1, 5, 10, 20 h)and 500° C.(1 h). Each test piece was inserted in an electric furnaceafter the furnace reached a predetermined temperature. The starting timeof oxidation occurred at the time of the test piece insertion into thefurnace. The SPCE steel has a tensile strength of 28 kg/mm² or more andan elongation 36% or more, at room temperature and in a rollingdirection.

Further, in order to examine the corrosion resistance of an oxide filmformed in a practical production process, an electric furnace was used,two sheets of SPCE steel plate, press-worked according to the practicalproduction process, were welded to form one pair, and ten pairs of suchsteel plate sheets were laminated and subjected to atmospheric oxidationtreatment. In this case, the temperature was raised after the test piecewas inserted, and the time when the temperature at the center of thefurnace reached to a predetermined temperature was determined to be thestarting time of the oxidation treatment.

A corrosion test solution was prepared by dissolving a commercial firstclass reagent LiBr.H₂O into ion exchange water, adjusting it to 65 wt %to cope with practical conditions, and then adding thereto, by weight %,LiOH (0–0.5%), Li₂MoO₄(0–0.035%) and LiNO₃ (0–0.005%). In particular, inthe present embodiment, a solution of LiBr 65 wt %, LiOH 0.3% andLi₂MoO₄ 0.02%, by weight %, was used as a corrosion solution and wastested at a temperature of 160° C.

A corrosion test of SS400 was conducted by putting test pieces and atest solution into an autoclave, deaerating it with argon gas for onehour, evacuating the autoclave to a pressure of 2 mmHg by a vacuum pumpand retaining the test pieces for 200 hours in the autoclave with thethermostat kept at 433° K.

In a corrosion test of SPCE, a glass tube was used, a test piece andtest solution were put into the tube, the tube was evacuated to apressure of 2 mmHg by connecting it with a vacuum pump and then it wasdeaerated under the condition of 2 mmHg and 25° C. The test piece washeld in the tube (thermostat) at 433° K for 200 hours.

After the corrosion test, corrosion products were removed from the testpiece, water-washed, dried and then measured for weight.

FIG. 2 shows the influence of atmospheric oxidation temperature upon thecorrosion amount after a 200 hour test of SS400. The corrosion amount ofthe test piece subjected to the atmospheric oxidation treatment of 300°C., 1 h scatters in a wide range of from 3.5 mg/dm² to a maximum of 67.5mg/dm². However, the maximum value of the corrosion amount is smallerthan that of a test piece which has not undergone the oxidationtreatment. In particular, the corrosion amount becomes 30 mg/dm² or lessat the oxidation temperature of 330° C. or more, more particularly 350°C. or more. In a case where the atmospheric oxidation was effected at atemperature of 400° C. or more, a variation in the corrosion amounthardly exists, and the corrosion amount was 10 mg/dm² or less and wasreduced to 1/10 or less. A lower limit value of the corrosion amount hasa tendency to rise as the oxidation temperature rises.

FIG. 3 shows the influence of the oxide film thickness produced byatmospheric oxidation on the corrosion amount of SS400. Here, theatmospheric oxidation oxide film thickness can be adjusted according tothe oxidation treatment time. When oxidation was effected for one hour,an oxide film which is about 1200 nm thick was formed on a surface. Theamount of corrosion of this test piece was 7.2 mg/dm², and the corrosionamount was reduced drastically by the oxidation. Even if the atmosphericoxidation time is made longer to further thicken the oxide film, theamount of corrosion almost never changes.

FIG. 4 shows changes, with the passage of time, in the corrosion amountof test pieces which have undergone and those which have not undergoneatmospheric oxidation. The corrosion amount of the oxidized test piecealmost never increases even after the passage of about 1000 hours.

FIG. 5 shows the dependence of a corrosion amount of a test pieceoxidized in an atmosphere on the Li₂MoO₄ concentration. The condition ofthe atmospheric oxidation was 400° C. for one hour. In FIG. 5, the testresult of a test piece not oxidized also is shown. The corrosion testcondition was the same as in FIG. 2. The condition of the atmosphericoxidation was 400° C. for one hour. In the case of no atmosphericoxidation, the corrosion amount rapidly decreases due to the addition ofan additive of Li₂MoO₄, and the corrosion amount decreases further byfurther increasing the additive. On the other hand, in a case whereatmospheric oxidation was performed, there was almost no dependency ofthe corrosion amount on the Li₂MoO₄ concentration, and no differenceoccurred between the addition and no addition of the additive.

FIG. 6 shows the dependence of a corrosion amount of a test pieceoxidized in an atmosphere on the LiOH concentration. The corrosionamount in a 65% LiBr solution without any additive was 500 mg/dm² ormore, which is very much, irrespective of whether there is oxidation orno oxidation. Even if LiOH is added, the corrosion amount of a testpiece on which no oxidation treatment was performed was reduced alittle, but a remarkable corrosion suppression was not observed. On theother hand, when the oxidation treatment was performed, the corrosionamount was rapidly reduced by adding 0.1% LiOH. However, the corrosionamount was hardly influenced by the LiOH concentration.

After a corrosion test was performed for 1000 hours, the condition of asection of a welded portion of each SPCE steel plate without atmosphericoxidation treatment and each SPCE steel plate with atmospheric oxidationtreatment at 400° C. for one hour was examined. In the case of the platewith no atmospheric oxidation treatment, pitting corrosion was observedin a deposition and heat-affected zone in some cases, but such pittingcorrosion did not occur in the test piece on which atmospheric oxidationtreatment was performed. In a case where oxidation treatment wasperformed at another oxidation temperature, no pitting corrosionoccurred, either.

The concentration of the inhibitor, after a corrosion test was carriedout for 1000 hours, was measured. In the case of no atmosphericoxidation treatment, Li₂MoO₄ was reduced from 0.02 wt % to 0.007 wt %and LiNO₃ was reduced from 0.005% to 0%. On the contrary, in a casewhere atmospheric oxidation treatment was carried out at 350° C. for onehour, Li₂MoO₄ was reduced from 0.02 wt % to 0.018 wt % and LiNO₃ wasreduced from 0.005% to 0.0028%. In particular, it was found that adecrease in the concentration of Li₂MoO₄ can be suppressed. This notesthe possibility that the interval for the addition of Li₂MoO₄ isextended, and means that an extension of the interval for maintenancealso is possible.

SEM photographs, obtained before the corrosion test, of SS400 testpieces on which atmospheric oxidation treatment was performed at 300,400 and 500° C. for one hour, were observed. Even if the test piece wassubjected to atmospheric oxidation at 300° C. for one hour, grinding orpolishing scratches were clearly observable on the surface and the oxidelayer was fairly thin. On the contrary, in the case of atmosphericoxidation at 400° C., needle-like and granular crystals were formed onthe surface, and cracks in the oxide also appeared on the surface. Whenthe oxidation was carried out at 500° C., the needle-like crystalsbecame less evident and the surface became uniformly covered withgranular crystals. After the corrosion test was carried out, in the casewhere the atmospheric oxidation treatment was performed at a temperatureof 300° C., an oxide layer was formed on the surface and grindingscratches became thin. In the test piece on which the atmosphericoxidation treatment was performed at a temperature of 400° C., theneedle-like crystals which appeared before the corrosion test becameless evident, however, the surface condition almost never changed. Inthe test piece on which the atmospheric oxidation treatment wasperformed at a temperature of 500° C., the needle-like oxides whichappeared before the corrosion test disappeared, and dense oxides underthe needle-like oxides remained. The oxide film formed by theatmospheric oxidation treatment at 400° C. or more was partiallydissolved by the corrosion test, but a large change did not appear onthe surface.

FIGS. 7, 8 and 9 each show X-ray analysis results for test pieces ofSS400 oxidized in an atmosphere for one hour at 300, 400 and 500° C. andthe results for test pieces after a corrosion test of the test pieces ina solution of 65% LiBr-0.3% LiHO-0.02% Li₂MoO₄ at 160° C. for 200 hours.In all cases, Fe₃O₄ and Fe₂O₃ were detected. As for the test piecesafter the corrosion test, it was considered that (LiFe₅O₈), (Li₅Fe₅O₈,FeO_(0.98), Li₂Fe₃O₅) and (Li₂MoO₃, Li₆Mo₂O₇) also were partiallydetected, other than Fe₃O₄ and Fe₂O₃. However, it is impossible toclearly separate them. The composition of an oxide film formed by anatmospheric oxidation treatment does not depend on the oxidationtemperature and is constant. The composition of an oxide film after thecorrosion test was basically the same as that before the corrosion test,which meets with the result of SEM observation.

FIG. 10 shows depth profiles of iron (Fe) and Oxygen by AES (AugerElectron Spectroscopy) after atmospheric oxidation treatment at 400° C.for one hour. The ratio between iron and oxygen was 1:1.2, which isclose to the ratio 1:1.33 of magnetite and meets with the result ofX-ray analysis. The thickness of the oxide film was about 1500 nm (1.5μm) according to the result of the AES. The thickness of the oxide film,obtained from washing a test piece with acid after oxidation treatmentand then detecting the thickness from its weight difference before andafter the washing, was about 1200 nm (the calculation is effectedassuming the oxide film is Fe₃O₄, and the density is 5.16 g/cm³ and itnearly meets with the result of the AES measurement, whereby it wasnoted that an oxide film formed by oxidation in an atmosphere is fairlydense.

Embodiment 2

FIG. 1 shows the construction of an absorption refrigerator representingan embodiment of this invention. Oxidation in this embodiment isperformed by recirculating high temperature air in a path including ahigh temperature regenerator 1→a heat exchanger 2→an absorber 3. Firstof all, the refrigerator, except for non-heat resisting parts, such aspumps, etc., are assembled temporarily, and then an air blower 6 with aheater 5 is connected to a gas introduction pipe 4 arranged in thevicinity of the high temperature regenerator 1. A valve 7A is closed andthen a valve 7B is opened, whereby heated high temperature air isintroduced into the refrigerator. The air used there has a moisture ofsteam partial pressure of 0.00782 (air with humidity of 25% at atemperature of 25° C.). In this case, the path including the hightemperature regenerator 1→a low temperature regenerator 8→a condenser 9(this is a line in which steam flows during operation of therefrigerator) is low corrosive, so that the necessity of preoxidationtreatment is small. Therefore, a valve 7C and a valve 7D are closed sothat high temperature air passes through the highly corrosive line ofthe high temperature regenerator 1→the heat exchanger 2→the absorber 3.The introduced air is introduced in the absorber through the highregenerator and the heat exchanger. The oxidation is carried out forabout 1–4 hours after structural material to be treated is raised to atemperature of 200–800° C. by air having a temperature of 200–800° C.

This closed recirculation type absorption refrigerator uses water as arefrigerant and a dense water solution of lithium bromide as anabsorption solution. The refrigerator comprises the high temperatureregenerator 1, the low temperature regenerator 8, a condenser 9, anevaporator 12, the absorber 3, pumps 13, 14 for recirculating theabsorption solution and the refrigerant therebetween, and the heatexchanger 2, and each apparatus operates as follows:

(A) High Temperature Regenerator 1

This device is used for heating to evaporate a refrigerant using flamesproduced by burning gas or oil. The vessel and the heat exchanger of theregenerator 1 each are made of carbon steel, and the bottom plate of afloat box therein is made of SUS 304 stainless steel.

(B) Evaporator 12

Cold water is passed through a bundle of pipes in the evaporator 12, arefrigerant is sprayed on the outside of the bundle of pipes, and latentheat of evaporation is extracted as heat from the cold water.

(C) Absorber 3

The lithium bromide water solution has a remarkably lower vapor pressurethan water of the same temperature and can absorb steam generated at afairly low temperature. In the absorber 3, the refrigerant evaporated inthe evaporator 12 is absorbed into the lithium bromide water solution(absorption solution) sprayed on the outer surfaces of the bundle ofpipes in the absorber 3. Absorption heat generated at this time isabsorbed by the cooling water flowing in the pipes.

The diluted solution which had absorbed the refrigerant in the absorber3 has its concentration lowered, and becomes to have a weak absorptionability. Therefore, a part of the diluted solution is transferred to thehigh temperature regenerator 1 by the solution pump 13 and is heatedtherein by high temperature steam, etc., whereby the refrigerant steamis separated from the steam by evaporation and the solution isconcentrated and returned to the absorber 3. Further, another part ofthe diluted solution from the absorber 3 is transferred to the lowtemperature regenerator 8 by the solution pump 13 and is heated andconcentrated therein by the steam of the refrigerant generated in thehigh temperature regenerator 1. The concentrated solution is returned tothe absorber 3. The steam of the refrigerant separated in the hightemperature regenerator 1 is cooled in the condenser 9 by cooling waterflowing in the pipe, is condensed and liquified, and then returned tothe evaporator.

(D) Heat Exchanger 2

A dilute solution at a low temperature flowing from the absorber 3toward the high temperature regenerator 1 is pre-heated by theconcentrated solution of a high temperature flowing from the hightemperature regenerator 1 toward the absorber 2 to reduce the amount ofheating in the regenerator.

(E) Solution Pump 13

The solution pump 13 circulates a rich solution, a lean solution andrefrigerant.

The absorber 3, the high temperature regenerator 1 and the solution pump13, as a whole, have the same function as a compressor in a compressiontype refrigerator. The absorption solution recirculates between the hightemperature regenerator 1 and the absorber 2 through the heat exchanger2 during operation of the refrigerator. The higher the concentration ofthe absorption solution is, the higher the efficiency of refrigerationis in general. Therefore, it is necessary to keep the high temperatureregenerator 1 at a higher temperature in order to concentrate theabsorption solution.

(F) Cooling Tower 19

The cooling tower 19 is used for cooling the cooling water issued fromthe condenser 9 with a coolant supplied from the outside of therefrigerator. The cooling water is sprayed and cooled by air blown by afan 18 driven by a motor (M), for example.

The piping in the vacuum vessel of the condenser 9, the low temperatureregenerator 8, the evaporator 12 and the absorber 3 each are made ofcopper, and the other pipings are made of carbon steel and are oxidizedso that they present one of the colors of blue, purple, (black) and greyrepresenting the thickness of the oxide film. The high temperatureregenerator 1 and heat exchanger 2 are made of carbon steel, and areoxidized so that oxide films of the same thickness and composition asmentioned above are formed. As a carbon steel, JIS structural rolledsteel SS400 is used. Mannesmann seamless pipes produced by hot workingare used for the piping arranged outside of the vacuum vessel, and blackoxide films having a thickness of 0.8–3 μm are formed on the surfaces ofthe pipes. The oxide film in this embodiment includes a hydroxyl groupor water and has an inner layer of magnetite and an outer layer ofhematite or its hydroxide.

FIG. 11 shows the amount of hydrogen gas generated by the corrosionliquid in the embodiment 1 during operation of the refrigerator in acase where oxidation is performed for 2 hours, the gas temperature at aninlet of the high temperature regenerator is adjusted to 800° C. and air(which is air having an arbitrary dew point with a humidity of 25% at25° C.) of a steam partial pressure of 0.00782 is used, and in a casewhere no oxidation is performed. Since the hydrogen gas is generated bythe corrosion reaction of iron, the amount of corrosion of the iron andan amount of hydrogen being generated are proportional to each other. Asshown in FIG. 11, the amount of hydrogen generated after operation ofthe refrigerator for 500 hours when the present invention is carried outis 1/20 or less, compared with a case wherein the present invention isnot employed. Further, in the present invention, the amount of hydrogengenerated almost never changes between 100 hours and 1000 hours, and ifthe change in the amount of hydrogen being generated in such a period isconverted into a hydrogen generation speed, it is 0.02 ml/minute. On thecontrary, when the present invention is not practiced or applied, theamount of hydrogen being generated increases between 200 hours and 1000hours, and the hydrogen generation speed is 2 ml/min, which is about 100times the hydrogen generation amount in the case where the presentinvention is practiced.

FIG. 12 shows the amount of consumption of an inhibitor in the operationof the refrigerator in cases where the present invention is practicedand not practiced. In the case where the present invention is practiced,the amount of consumption of an inhibitor after operation for 1000 hoursis very small and, in particular, the inhibitor is not consumed at allafter 600 hours. On the contrary, in the case where the presentinvention is not practiced, the inhibitor is reduced to about half orless after operation for 500 hours, and continues to decrease afteroperation for 1000 hours. Therefore, it is apparent that the number oftimes the inhibitor needs to be supplemental can be reduced extremelywhen the present invention is applied. By practicing the presentinvention, the corrosion, the gas generation amount and the inhibitorconsumption amount can be suppressed to very small values, whereby theperformance, reliability and durability of the refrigerator can beraised.

Embodiment 3.

In another preoxidation treatment method, a heating source used in thehigh temperature regenerator is used as a heat source for the oxidationtreatment to form the same oxide film as in the embodiment 1. In thiscase, the heater 5 as shown in FIG. 1 is unnecessary. By operating a gasblower under a condition in which the interior of the high temperatureregenerator is heated to a prescribed temperature by using the heatingsource of the high temperature regenerator, high temperature air can besupplied to the interior of the refrigerator, whereby an oxide film canbe formed on the inner surfaces of the structural material of therefrigerator.

Embodiment 4

FIG. 13 shows a steam generator of the type used when a corrosionprotective film is formed using steam. Air or oxygen transferred by thegas blower 6 is bubbled in water within the steam generator (A). Theinterior of the steam generator is provided with a heat source (B) andis under a pressurized condition. The gas being bubbled becomes a gascontaining a lot of steam in the steam generator (A), and the gas isheated to an arbitrary temperature by the heater 5. By connecting thesteam generator to the valve 7B shown in FIG. 1 through the gasintroduction pipe 4, a corrosion protective film can be formed usingsteam. In any of the methods, since high temperature gas is recirculatedby the blower, etc., a temperature drop occurs even if a heat insulatoris provided. Therefore, it is better if the temperature of the gas to besupplied is higher. In this embodiment, the gas temperature is set to800° C., and, as a result, it is 400° C. at the absorber positioneddownstream of the position from which the gas is blown, which is asufficient temperature to effect the oxidation treatment. In thismethod, since high temperature gas is not directly introduced into thecondenser, but all of the interior of the system is raised to a hightemperature, the condenser is heated to 300° C. by the extra heat. Thetemperature is sufficient to effect the oxidation treatment thereof aswell as the absorber, and an oxide film having a desired color can beformed in the same manner as in the embodiment 1.

The oxide film formed by the atmosphere including steam is high inwetability with respect to the refrigerant, and a high efficiency ofcooling can be attained.

Embodiment 5

In this embodiment, instead of utilizing the air blower 6 with the gasintroduction pipe 4 and heater 5 in the embodiment 2, the wholerefrigerator can be subjected to 2 hours of heat treatment (P=4.76×10³)by an electric furnace at 400° C. in the air (steam pressure 14.255mmHg, steam partial pressure 0.01875) including steam of 20° C. and witha humidity of 60% in a stage before filling the absorption solution andrefrigerant and after temporarily assembling the absorptionrefrigerator, except for non-heat resisting apparatuses, such as pumps,regulation valves, etc. The corrosion amount, hydrogen gas generationamount, and inhibitor consumption amount of the refrigerator in whichthe oxidation treatment according to the present invention is performedcan be suppressed to very small values in the same manner as in theembodiment 1, whereby the performance, reliability and durability of therefrigerator can be raised. In this method, since the whole refrigeratoris heated, a highly adhesive oxide film can be coated all over the fineportions. Further, since the whole refrigerator is heated, it ispossible to coat corrosion protective films on all of the portions whichcome in contact with liquid, so that corrosion resistance can be raisedremarkably.

In a case where steam is used, corrosion protective films can be formedby providing the steam generator as shown in FIG. 13 on the refrigeratordisposed in the electric furnace.

Embodiment 6

FIG. 14 shows a corrugated plate heat exchanger. The heat exchanger usesa 0.5 mm thick plate which is made of carbon steel SS400 or SPCE steel.Two sheets, each of which is corrugated by a press, are piled up andwelded at the peripheral portions thereof to be formed in a bag-shapedheat conductor. A plurality of the bag-shaped heat conductors arelaminated to form a heat conductor group. In FIG. 14, it is seen that anoblique line part is corrugated and another portion is flat withoutcorrugations. Piping is connected to portions shown by circles (◯). Theheat conductor group is accommodated in a box, as shown in FIG. 15.

FIG. 15 shows a part of a section of the plate heat exchanger, that is,a construction formed by a lamination of the corrugated plates. In thisconstruction, a liquid inlet and a liquid outlet are connected as shownin FIG. 14 so that A-liquid flows in portions of odd number layers, suchas a first layer, a third layer, a fifth layer . . . , and B-liquidflows in portions of even number layers, such as a second layer, afourth layer, . . . , in parallel to each other. Inlets and outlets forthe A-liquid and B-liquid are arranged oppositely to each other as shownin FIG. 14, and the A-liquid and B-liquid are separated from each otherby the bag-shaped heat conductors as shown in FIG. 15. The bag-shapedheat conductors have their respective inlets and outlets for theB-liquid, connected to pipes so that the B-liquid within the bag-shapedheat exchangers are joined through the pipes. The heat exchanger isconstructed so that the A-liquid and B-liquid flows in oppositedirections, and has a high efficiency. Further, the A-liquid flowsoutside the bag-shaped heat conductors and an inlet and outlet areprovided for the A-liquid.

The A-liquid is a water solution including a high concentration halogencompound issued from the high temperature regenerator of FIG. 1, and theB-liquid is a water solution including a low concentration halogencompound issued from the absorber of FIG. 1.

Since the plate heat exchanger has a large surface area with a smallsize, the thermal efficiency is very high; on the other hand, asignificant amount of hydrogen gas is generated by corrosion because ofthe large surface area, whereby the refrigeration performance isreduced. Further, each sheet of the plate is made very thin, such as 0.5mm or less, in order to raise the thermal efficiency, so that it is veryimportant to suppress corrosion in this structure.

The corrugated plate type heat exchanger is disposed under a verycorrosive condition in which a plurality of absorption solutionsdifferent in temperature and concentration mixedly exist. Irrespectiveof such condition, a very thin steel plate of 0.4–0.5 mm is used for theheat exchanger in view of the heat conduction characteristic of theplate. If a hole or holes are formed in the plate by corrosion, the heatconduction characteristic will be reduced drastically, so that it isvery important to prevent corrosion on this portion.

FIG. 16 shows a flow chart of a method of production of the corrugatedplate type heat exchanger. A steel plate is cut off to a predeterminedsize, and then it is pressed by a press to form corrugations thereon.After decreasing, a predetermined number of the plates are laminated andthen welded at their peripheral portions to join them, whereby the heatexchanger is formed. The heat exchanger is inserted in an electricfurnace in air in the same manner as in the embodiment 1. The heatexchanger is heated for a predetermined time in flowing air caused bythe fan provided in the electric furnace. After the oxidation, the colorof the surface is observed, and if the color is in a range from a blueinterference color to black, inclusive of grey, the work is advanced tothe next step. In this step, it is judged whether or not the formed filmis sound, that is, whether the film has a thickness presenting asufficient corrosion resistance. When the formed oxide film is too thin,the diffusion suppression ability for a substance susceptible to acorrosion reaction is low.

As a method of examining the thickness of an oxide film, there is aninstrumental analysis, such as an Auger spectroscopic analysis. Althoughit is difficult to use such an analysis for a production line, since theformed oxide film is thin, it is possible to judge the appropriatethickness thereof by its color. After oxidation treatment is performedby setting the temperature and time to various values, the relationbetween the color of the surface of the film and the thickness of theoxide film can be determined by the Auger spectroscopic analysis. As aresult, the following relations were obtained by heating a test piecefor one hour at each of the following temperatures: at 200° C. a darkmetal surface 30

-   A (angstrom), at 300° C. (P=3.88×10³) blue interference-   color 300 A, at 400° C. (P=4.56×10³) purple interference-   color 2000 A, at 500° C. (P=5.24×10³) grey 6000 A, at 650° C.    (P=6.26×10³) black 12000 A or more. Accordingly, in a case where the    color of the surface is a dark metal surface color, the oxidation is    insufficient as yet, so that the oxidation should be further    continued while studying the oxidation temperature or time, and a    measurement, such as the elevation of the oxidation temperature,    should be monitored. This embodiment is described with reference to    a case where a closed type electric furnace is used, the entire    corrugated plate type heat exchanger is inserted in the electric    furnace, and the same air as in the embodiment 1 is used as a gas,    whereby an oxide film is formed. In a case where the oxidation    temperature and time are set at 150° C. and 1.0 hour (P=2.87×10³)    (the oxidation time represents time for which surface is exposed to    the temperature as shown in FIG. 17), since the color of the surface    after the oxidation is a dark metal surface color, further oxidation    should be performed for one hour, while raising the temperature to    300° C. As a result, the color of the surface will turn to a blue    interference color.

FIG. 18 shows the relation between the oxidation treatment temperatureand the oxide film thickness.

In the next step, whether or not the oxide film is separated from thesurface is studied. There is not necessarily a relationship between thethickness of the oxide film and the corrosion. In a case where thethickness is excessive, cracking or boundary separation is apt to occurin the oxide film by residual stress at the time of the film growth andat the time of cooling after preoxidation treatment, and the oxide filmno longer can serve as a corrosion protective film. For example, in acase where a carbon steel member is heated to 1000° C., a black oxidefilm is formed on the surface, however a part of the film is separatedand the base metal color is confirmed at the separated portion. Sincesuch an oxide film has many cracks formed therein, it can not serveeffectively as a corrosion protective film. However, when the oxidationis effected at a temperature of 600° C., separation of the oxide film isnot observed. One of the causes which influence separation of the oxidefilm is the rate of temperature increase. FIG. 17 shows temperaturecharacteristics when heat treatment is performed on carbon steel, usingthe same air as in embodiment 1. Cooling is effected in the furnaceafter oxidation treatment. In the case where the oxidation temperatureand the rate of temperature increase are 300° C. and 300° C./h,respectively, as expressed by a characteristic curve (1), the color ofthe surface is a blue interference color and no separation of the oxidefilm is observed. However, in the case where the oxidation treatmenttemperature and the temperature rising rate are 500° C. and 300° C./h,respectively, as shown by a characteristic curve (2), the color of thesurface is grey, but separation of the oxide film at a part of thesurface is observed. Therefore, the rate of temperature increase is setto 250° C./h, as shown by a characteristic curve (3), and, as a result,no separation of the oxide film is observed. In the case where theoxidation temperature is more than 500° C., also, the result is the sameas the case where the oxidation temperature is 500° C. Therefore, whenair is used, the rate of temperature increase is suitable at about 300°C./h at an oxidation temperature of 300° C., and at about 250° C./h atan oxidation temperature of 500° C. or more. When steam is used,separation of the oxide film is not observed even at an oxidationtemperature of 500° C. or more and a rate of temperature increase of300° C./h. When no separation of the oxide film is observed, the processis advanced to the step of assembling the absorption refrigerator. Inthis embodiment, cooling in the furnace is effected after the oxidationtreatment; however, the same effect also can be attained by cooling inair after oxidation.

The plate heat exchanger has a construction in which a pair of (two)plates are welded by plasma at their peripheral portions, and aplurality of the pairs (for example, 10 pairs) of plates are piled upand enclosed in a box. The construction is oxidized in an atmosphere ata predetermined temperature for one hour.

FIGS. 19–21 show the corrosion amounts, with the passage of time, oftest pieces cut out from a plate heat exchanger which was produced usingSPCE steel in a practical process and oxidized in an atmosphere atrespective temperatures in a range of 300–450° C. for one hour. Thecorrosion reagent used there was the same as in the case of thecorrosion estimation of SPCE welding portions in the embodiment 1. Eachnumber represents data for a test piece cut out from a plate identifiedby a plate number, such as shown in FIG. 15. As for No. 8-1, there wasalmost no effect of oxidation treatment and the corrosion amount wasalmost the same as that in the case of no oxidation treatment. When theatmospheric oxidation treatment temperature was at 350° C., there wasalmost no variation in the corrosion amount, and the corrosion amountwas reduced to 1/10 or less as compared with the case in which noatmospheric oxidation was performed. At 400° C. and 450° C., the casewas the same as at 350° C. Further, the higher the atmospheric oxidationtemperature was, the more the corrosion amount became, as in case ofSS400. Particularly, as shown in FIGS. 20 and 21, the material treatedaccording to the present invention presents such an excellent propertythat the corrosion amount even by immersion in a corrosive solution for1000 hours was 20 mg/dm² or less, in some cases 10 mg/dm² or less.

Embodiment 7

FIG. 22 shows the construction of a high temperature regenerator. Anabsorption solution diluted in the absorber is transferred to the hightemperature regenerator, wherein the solution is heated and concentratedby burners (in case of direct boiling). The heated concentratedabsorption solution is transferred to the high temperature heatexchanger. On the other hand, a refrigerant vapor generated in the hightemperature regenerator is transferred to the condenser through a mistseparator.

In the high temperature regenerator, since the absorption solutionreaches a high concentration and high temperature, such as LiBr at 65%and 160° C., remarkable corrosion takes place here without any corrosionpreventing treatment, so that it is necessary to effect corrosionprevention. In this embodiment, also, the entire high temperatureregenerator was inserted in the closed electric furnace and oxidationtreatment was conducted, in the same manner as in the embodiment 6.

In the absorption refrigerator in which oxidation treatment wasconducted on the structural components or apparatuses in this manner,and then assembled, the amount of hydrogen generation and the inhibitorconsumption amount were the same as the results of FIGS. 11 and 12 inwhich the oxidation treatment was effected after assembling therefrigerator, and so effective corrosion resistance and good reliabilityof the refrigerator can be secured. The oxidation treatment was carriedout after the burners were taken out. The pipes of the vessel and theheat exchanger portion and the tube plates for connecting the pipes eachare made of carbon steel. In this embodiment, the film thickness ispreferably deep purple in color and 3000–4000A thick.

In the inspection method applied here, outside surfaces can beinspected, since the entire refrigerator is inserted in the electricfurnace after the structural components are assembled. In case of theembodiment 5, also, since the entire refrigerator is inserted in theelectric furnace, inspection can be carried out by observation of theouter surface of the refrigerator. On the contrary, in the case of theembodiment 2, since high temperature gas is introduced in the interiorof the refrigerator, an inspection is desirable to observe the interior.In this regard, it is possible to inspect the interior by using afiberscope, etc. Further, since the outside surface also is heated byintroduction of a high temperature gas into the refrigerator, theoutside surface is oxidized by air coming in contact therewith, so thatthe inspection of the interior can be replaced by inspection of theoutside surface.

Further, the above-mentioned pipe is provided with a lot of ring-shapedfins of copper, and oxide films also are formed on the surfaces of thefins.

Embodiment 8

The present embodiment relates to a method of preoxidation treatment ofpiping 21. In case of oxidation treatment of the piping 21, it isunnecessary to insert the entire piping 21 in the electric furnace 22.The piping 21 is passed through an electric furnace of a certain lengthand is moved therethrough at a fixed speed, as shown in FIG. 23, wherebythe oxidation treatment can be effected on the inner surface of thepiping. As shown in FIG. 24, the inner surface of the piping can betreated with oxidation by inserting a columnar heater 23 in the piping21, which is fixed in position, and then moving the heater relative tothe fixed piping 21.

Embodiment 9

oxidation treatment for one hour at each of 100, 200, 300, 400, 600 and800□C under the same air conditions as in the embodiment 2 was effectedon SS400 carbon steel as the structural material of the refrigerator.The surface color of the material after the treatment was a dark metalcolor at 200° C., a blue interference color at 300° C., a blue purpleinterference color at 400° C., a bluish black interference color at 600°C., and black at 800° C. The carbon steel on which the oxidationtreatment was performed in this manner was immersed in a water solutionof LiBr 50–70 wt % and 160° C. for 1000 hours, which is a most severecondition for the absorption refrigerator. An inhibitor of LiOH 0.05–1.0wt %, Li₂MoO₄ 10–150 ppm (as MoO₄ ²⁻), LiNO₃ 5–350 ppm (as NO₃ ⁻)coexists with the absorption solution. A true oxide film amount wasobtained by an oxide film formed by subtracting the oxidation treatmentbefore the immersion from an appearance corrosion amount obtained byremoving the oxide film existing on the surface.

FIG. 25 shows the relation between the corrosion amount and time. InFIG. 25, a case where the oxidation treatment was not utilized also isshown. The corrosion amount at an oxidation temperature of 100° C. wassubstantially the same as in the case where no oxidation treatment wasperformed. When the oxidation treatment temperature was raised to 200°C., the corrosion amount was reduced to about a half. Further, when theoxidation treatment temperature becomes 300° C. or more, the corrosionamount can be reduced to about 1/10 or less, and after the passage of600 hours, corrosion hardly develops. In the embodiment shown in FIG. 6,air (humidity 65% at 28° C.) was used, the atmosphere of which was notcontrolled, that is, the air had a vapor pressure of moisture at anarbitrary dew point. The property of an oxide film after the oxidationtreatment and before the corrosion test was inspected by using an X-rayanalyzer, and, as a result, an oxide film could not be detected for thecase of oxidation treatment at a temperature of 100° C. and the casewithout oxidation treatment. Magnetite and hematite were detected at anoxidation treatment temperature of 200° C. or more. By an XPS (X-rayphotoelectron spectroscopy) measurement, trivalent iron was detected onthe surface, from which it is found that the film structure was acompound oxide film comprising an inner layer of magnetite and an outerlayer of hematite. Further, in the same manner, the existence of watermolecules or a hydroxyl group on the surface of carbon steel wasconfirmed by the XPS measurement. On the contrary, when an oxidationtreatment was effected, using dry air (oxygen concentration of 10 ppm orless), the existence of water molecules or a hydroxyl group was notconfirmed.

Further, in FIG. 25, a high corrosion resistance appears when oxidationtreatment is effected at 400° C. or more; however, in the structuralmembers treated with oxidation at 600° C. or more, separation of theoxide film was observed, and the corrosion amount increased more.Further, it was observed that in some cases, the corrosion amount was alittle larger than the characteristic line.

Embodiment 10

Corrosion test results for carbon steel (SS400) and stainless steel (SUS304) are shown in Table 2. The corrosion condition was the same as inthe embodiment 8. As for carbon steel, in the case where oxidationtreatment was performed, using the same air as in the embodiment 2 as anoxidizing atmosphere, at an oxidation temperature of 300° C., anoxidation time of 0 hour (cooling starts when the temperature reaches300° C.), and at a rate of temperature increase of 300° C./h, thecorrosion amount was reduced to about a half as much as that in case ofan oxidation treatment of one hour at 200° C. When the oxidation time isone hour, the corrosion amount can be reduced to about 1/10 as mentionedabove. In the case where the oxidation temperature was 400° C. and theoxidation time becomes 4 hours, the corrosion amount differs from thecase where the oxidation temperature is 300° C. and the oxidation timeis 0 hour, but can be reduced to 1/10, which is the same as the case ofan oxidation temperature of 300° C. and an oxidation time of 0 hour.This is because the time for which an object to be treated is exposed atemperature of to 300° C. is 1 hour or more. As mentioned above, asimilar good corrosion resistance can be attained even at the oxidationtemperature of 800° C. However, according to a Fe—Fe₃C phase diagram,transformation occurs at 723° C. Accordingly, in case of oxidationtreatment of thin plate, 300–500° C. and 1–4 hours are most suitable.However, in absorption refrigerator structural components havingcomplicated portions, such as gaps, there typically will be a portion orportions which rapidly lack exposure to which oxygen in the air. Inoxidation treatment of such structural components, it is desirable tooxidize them at a relatively high temperature, such as 500–800° C., topromote diffusion of oxygen in the air. In the case of a thick plate,there is little deformation by transformation, and there is no problemin working it.

In oxidation treatment of carbon steel at a temperature of 500° C. ormore, it is easy to control the atmosphere, heating rate, cooling rate,etc. in the case of a small test piece, and so a uniform oxide film canbe formed. However, in the oxidation treatment of practical products,such as a large-sized or complicatedly shaped heat exchanger, hightemperature regenerator, etc., separation takes place in the oxide film,and a uniform film is not readily formed, so that the oxidationtemperature should be 300° C.-less than 500° C., preferably, 380–470°C., and more preferably 400–450° C.

In the oxidation treatment examples shown in FIGS. 23 and 24, theoxidation was effected using air (humidity 65% at temperature 28° C.)having a vapor pressure of the moisture in the atmosphere of anarbitrary dew point. Therefore, when the dew point is controlled, theapparatus and work are very complicated, which is a disadvantage.However, when the oxidation is effected in air, the apparatus andprocess are very simple, so that the latter oxidation treatment isexcellent as compared with the former.

When the oxidation is effected in the atmosphere of steam, the amount ofcorrosion becomes minimum at about 500° C., so that oxidation of thethin plate is preferably effected at a temperature of 400–600° C. In thecase where steam is used, the wetability of the surface is excellent,compared with a metal surface not treated with oxidation and a surfaceoxidized with air, and application of this oxidation method to anabsorption refrigerator can raise the refrigeration efficiencyremarkably. Therefore, steam is suitable for the oxidation treatment ofa part or an apparatus whose wetability is problematic, such as anabsorber, and air oxidation is suitable for other parts or apparatus. Itis confirmed by XPS measurement that hydroxyl groups or water moleculesexist on the surface of a test piece on which the steam oxidation iseffected, as in the case where oxidation is effected in air at anarbitrary dew point.

As for stainless steel treated with oxidation, the occurrence ofcorrosion is approximately the same as that of carbon steel, however,the amount of corrosion is ½–¼ times the corrosion amount in carbonsteel, as a whole. The test was conducted on SUS 304 stainless steel ofaustenitic stainless steel. However, similar results were attained onferritic stainless steel and low alloy steel. Stainless steel,particularly, austenitic stainless steel, is sensitized when it isexposed to a temperature of 500–900° C. for a long time. The sensitivityis small in a short time of one hour. However even in such a short time,the temperature region most suitable for members stressed thereon is300–500° C.

Embodiment 11

FIGS. 26–31 show the construction of an absorption refrigerator. FIGS.27 and 28 are left and right side views, respectively. FIG. 29 is aperspective view of the whole structure. FIG. 30 is a system, includinga cooling water line and a cold water line, connected to the apparatusof FIG. 29, and FIG. 31 also is a steam line connected to the apparatusin FIG. 29.

The absorption refrigerator of this embodiment has a construction asshown in FIG. 1. In this embodiment, a heat exchanger 2 is constructedof corrugated plates as shown in FIGS. 14 and 15. A high temperatureregenerator 1 is constructed as shown in FIG. 22. Each apparatus issubjected to the same oxidation treatment as in the embodiments 4 and 6before being assembled. The main structural material of each apparatusis SS400 carbon steel, and oxidation is effected in air with a humidityof 60% at 25° C., at an oxidation temperature of 450° C., and retainsthe temperature time of one hour and a rate of temperature increase of300° C./h. As a result, an oxide film about 0.4 μm thick is formed. Theoxide film presents a deep purple interference color. Further, pipingconnected to the cooling water line and cold water line in FIG. 30 andthe steam line in FIG. 31 also are made of carbon steel pipe. A blackoxide film having a thickness of 1–5 μm is formed on each surface. Thepiping on which an oxide film is already formed is obtained by purchase,or the oxidation treatment is effected on non-oxidation treated pipingunder the condition, using the same air as mentioned above, of anoxidation temperature of 650° C., a retaining time of one hour and atemperature rising rate of 300° C./h.

Using the absorption refrigerator of this embodiment, in which there wasemployed an absorption solution having LiNO₃ 350 ppm as NO₃ ⁻ added to acommercial absorption solution (LiBr solution) including lithium bromideof 65 wt % and lithium hydroxide of 0.15 wt %, the refrigerator wasoperated at full load for 100 hours, then Li₂MoO₄ 75 ppm as MoO₄ ²⁻ wasadded into the absorption solution to form a compound inhibitorcontaining solution, and under this condition the refrigerator wasoperated at full load for 100 hours. As a result, the amount of hydrogengas generated in the refrigerator was very little.

According to the present invention, it is possible to increase thecorrosion resistance and reliability of the product by forming inadvance a film of a particular color having a good corrosion resistanceand satisfactory adhesion on the surfaces of structural materials for anabsorption refrigerator.

1. A production method of an absorption refrigerator using a refrigerantand its absorption solution, comprising: heating a surface of at least apart of a heat exchanger and high temperature regenerator in anatmosphere containing oxygen gas to oxidize the surface so as to form anoxide film having a thickness of 0.02–5.0 μm thereon, said oxide filmbeing in contact with said absorption solution and protecting saidsurface from corrosion due to said absorption solution during operationof the absorption refrigerator.
 2. A production method of an absorptionrefrigerator using water as a refrigerant and a halogen compound as itsabsorption solution, comprising: heating a surface of at least a part ofa heat exchanger and high temperature regenerator in an atmospherecontaining oxygen gas to oxidize the surface so as to form an oxide filmhaving a thickness of 0.02–5.0 μm thereon, said oxide film being incontact with said absorption solution and protecting said surface fromcorrosion due to said absorption solution during operation of theabsorption refrigerator.
 3. A production method of an absorptionrefrigerator using water as a refrigerant and a halogen compound as anabsorption solution, said absorption refrigerator comprising a hightemperature regenerator heating a water solution containing therein thehalogen compound to generate steam, a condenser condensing the steam, alow temperature regenerator cooling the steam, an evaporator evaporatingthe water from said condenser and generating cold water, an absorberabsorbing the water from said evaporator into the water solutioncontaining therein halogen compound of high concentration, and a heatexchanger returning the refrigerant from said absorber to said hightemperature regenerator and exchanging heat between the water from saidlow temperature regenerator and the refrigerant from said absorber, saidproduction method comprising: heating a surface of at least one of saidhigh temperature regenerator, said low temperature regenerator, saidabsorber and said heat exchanger in an atmosphere containing oxygen gasthereby to oxidize the surface so as to form an oxide film having athickness of 0.02–5.0 μm thereon, said oxide film being in contact withsaid absorption solution and protecting said surface from corrosion dueto said absorption solution during operation of the absorptionrefrigerator.
 4. A production method of an absorption refrigerator usingwater as a refrigerant and a halogen compound as an absorption solution,said absorption refrigerator comprising a high temperature regeneratorheating a water solution containing therein the halogen compound togenerate steam, a condenser condensing the steam, a low temperatureregenerator cooling the steam, an evaporator evaporating the water fromsaid condenser and generating cold water, an absorber absorbing thewater from said evaporator into the water solution containing therein ahalogen compound of high concentration, and a heat exchanger returningthe refrigerant from said absorber to said high temperature regeneratorand exchanging heat between the water from said low temperatureregenerator and the refrigerant from said absorber, said productionmethod comprising: heating said heat exchanger in an atmospherecontaining oxygen gas to oxidize a surface of said heat exchanger so asto form an oxide film having a thickness of 0.02–5.0 μm thereon, saidoxide film being in contact with said absorption solution and protectingsaid surface from corrosion due to said absorption solution duringoperation of the absorption refrigerator.
 5. The production methodaccording to claim 4, wherein said heating is carried out at atemperature of 330–500° C.
 6. The production method according to claim4, wherein said heating is carried out at a temperature of 350–400° C.7. The production method according to claim 3, wherein said heating iscarried out at a temperature of 330–500° C.
 8. The production methodaccording to claim 3, wherein said heating is carried out at atemperature of 350–400° C.
 9. The production method according to claim2, wherein said heating is carried out at a temperature of 330–500° C.10. The production method according to claim 2, wherein said heating iscarried out at a temperature of 350–400° C.
 11. The production methodaccording to claim 1, wherein said heating is carried out at atemperature of 330–500° C.
 12. The production method according to claim1, wherein said heating is carried out at a temperature of 350–400° C.13. The production method according to claim 4, wherein said oxide filmhas a thickness of 0.1–2.5 μm.
 14. The production method according toclaim 4, wherein said oxide film has a thickness of 0.3–2.0 μm.
 15. Theproduction method according to claim 3, wherein said oxide film has athickness of 0.1–2.5 μm.
 16. The production method according to claim 3,wherein said oxide film has a thickness of 0.3–2.0 μm.
 17. Theproduction method according to claim 2, wherein said oxide film has athickness of 0.1–2.5 μm.
 18. The production method according to claim 2,wherein said oxide film has a thickness of 0.3–2.0 μm.
 19. Theproduction method according to claim 1, wherein said oxide film has athickness of 0.1–2.5 μm.
 20. The production method according to claim 1,wherein said oxide film has a thickness of 0.3–2.0 μm.